1. Field of the Invention
The present invention relates to semiconductor devices and control methods of the same, and more particularly, to a semiconductor device having an memory cell array in which memory cells for storing data are included and a control method of the semiconductor device.
2. Description of the Related Art
In recent years, a memory device or the like has increasingly had a protection mechanism to prevent the program from tampering. The protection methods are to include a content addressable memory (CAM) and to include a volatile bit (latch). The CAM is a non-volatile memory that stores whether or not the content is addressable. In either method, the protection can be performed by every block such as a sector.
If the protection is performed with the latch, all the protection information has to be set or reset in a latch circuit at the time of power on. Depending on the user, the latch should have a protection status in some cases, and the latch should not have the protection status in other cases. Therefore, the CAM is demanded to selectively change and satisfy the both cases in the circuit design.
Conventionally, the function has been selectively changed according to the CAM information as a general method. Setting or resetting the latch circuit should be performed only once after the power is on. Hence, it is not preferable that all the blocks be set or reset after the user has set the protection information of (protected) a certain block in the latch circuit. In a conventional circuitry, the information of the CAM is read and all the blocks are set or reset in the latch circuit with a signal that becomes activated after a power level is detected to have exceeded a certain level, for example, 2 V, at the time of power on.
With this method, however, the CAM has to be read to set or reset the latch circuit in an unstable condition that a power supply voltage is considerably lower than the normal operation. A sufficient level of voltage has to be supplied to a gate of a cell transistor to read the CAM. It is difficult to read the CAM correctly, unless the sufficient level of voltage is not supplied to the gate. Also, a power-on speed varies depending on the user, and there is a drawback in that it is difficult that the operation is performed certainly under every condition.